1. Field of the Invention
The present invention relates to a valve device of a storage tank for transporting oils or chemical compounds, and more particularly, to a valve for maintaining the pressure of the storage tank to be always safe by preventing supersaturated pressure or negative pressure which is the opposite phenomenon of the supersaturated pressure, the supersaturated pressure being generated in the storage tank of transportation vessels such as an oil tanker. Also, the valve device in the present invention prevents the entry of flames into the storage tank from outside the storage tank.
2. Description of the Related Art
Usually, supersaturated pressure and negative pressure (vacuum phenomenon) are generated frequently in storage tanks for storing various oils or chemical compounds. The pressure in a storage tank can be increased by volatile gases from various oils or chemical compounds stored in the storage tank. Pressure in the tank can also vary due to changes in temperature in different climates in each district when an oil tanker or a transportation vessel of chemical compounds sails in the wide ocean. In almost all such cases, supersaturated pressure is generated. If an oil or a chemical compound is discharged from the storage tank by pumping, the pressure is lowered in the storage tank and negative pressure (vacuum phenomenon) is caused.
If the pressure is changed in a closed space, not only oils or chemical compounds which are highly explosive and inflammable become unstable, but also here is a high probability for an explosion or fire to occur. A serious accident results if such a storage tank explodes. Therefore, every storage tank for storing oils or chemical compounds is equipped with vent pipes which are connected to the outside at all times. However, since these vent pipes are always open, a large amount of oils or chemical compounds are evaporated through them. This eventually causes economic loss as well as contaminated air.
There have been a few inventions to resolve the above-identified problems. They include Korean Utility Model Registration No. 125817 entitled "A valve device for absorbing or discharging air of a transportation tank of oils or chemical compounds" owned by the applicant of the present invention, and U.S. Pat. No. 5,060,688. In these invention, however, suction means for external air into the storage tank and discharging means for expanded gases which discharge volatile gases from the storage tank, are separated from each other, crossing at a right angle and are connected to each other by means of other connection pipes. They are therefore disadvantageous in that they occupy a large installation space. Further, there is a problem of increasing the weight of a vessel if the above conventional equipments are installed in a transportation vessel since their own weight is considerable. Since more than ten of these devices are installed in one vessel, their weight is not insignificant. Further, it has been inconvenient to repair them since their unit price has increased and their structure is unreasonably complicated. Still further, it has been difficult to clean foreign materials trapped inside the device when it is in use.